This invention relates generally to centrifugal compressor wheels or impellers and, to method for making same. These wheels are commonly used in turbochargers, superchargers, and the like. More specifically, this invention relates to an improved compressor wheel utilizing a hardened, internally threaded nut made of comparatively stronger material and located within the wheel itself.
So called boreless compressor wheels, such as the ones which are the subject of copending U.S. Pat. No. 4,705,463 entitled "Compressor Wheel Assembly for Turbochargers" and naming Fidel M. Joco as inventor, are able to rotate at higher speeds than compressor wheels having a through bore since more of the wheel material exists at the geometric center of the wheel. That is, more wheel material exists at the point of maximum centrifugal load. It is recognized that the portion of the compressor wheel at the axis of rotation and axially aligned with the radially outermost portion of the wheel experiences the maximum centrifugal load.
It is also recognized that the portion of the compressor wheel which aligns with the radially outermost portion of the wheel needs to be solid and free of stress raisers such as through bores in order to hold the wheel together, and yet provision must be made to attach to a shaft. Thus, a design conflict is faced by designers of compressor wheels. In conventional compressor wheels the existence of a through bore in the central hub section, from which the blades extend and exert their centrifugal force, weakens the wheel. Such a weakened wheel cannot be operated at as high a rotational speed as a wheel which does not have a bore.
In recent years, improvements in compressor technology and design have resulted in progressive increases in compression efficiencies and flow ranges, together with more rapid response characteristics. Compressors wheels for turbochargers have highly complex blade surfaces that are designed for optimum efficiency at high speeds. Such blades are most advantageously and economically made by utilizing a lightweight material, such as aluminum or aluminum alloy. Aluminum and its alloys are chosen for their relatively low rotational inertia in order to achieve a further advantage of rapid acceleration during the transition from slow to fast operating speeds.
However, it is because these wheels are cast of aluminum or aluminum alloy that they are susceptible to failures in response to tensile loading when a central bore is present. But, in the absence of a complete bore through the wheel, means must nevertheless be provided for attachment of a shaft to the wheel. The placement of threads in the comparatively soft aluminum alloy material, for direct attachment to the shaft, has not proved strong enough to withstand the high torque stresses to which the soft alloy threads will be subjected during operation. One method to avoid the above problem is outlined in the aforementioned patent application: U.S. Ser. No. 873,265, wherein a relatively harder and stronger metal piece defining the threads is friction welded onto the back of a relatively softer compressor wheel. However, frictional welding of two metals of dissimilar materials creates a less than optimal bond strength. Even though the completed compressor wheel does have a blind bore defined by the welded thread-defining piece, these wheels are referred to generally as being "boreless". Although this term is strictly incorrect, it distinguishes those compressor wheels having a through bore from those wherein the more highly stressed portion of the wheel is not subject to the disadvantage of such a bore.